The electrophoretic effect operates on the principle that when electrophoretic particles are electrically charged to a particular polarity, the charged electrophoretic particles will migrate from a surface being charged to the same polarity as the charged particles toward a surface charged to a polarity opposite to that of the charged particles. For example, particles which are positively charged will migrate from a positively charged surface toward a negatively charged surface.
Display devices that utilize the electrophoretic effect are known as electrophoretic image displays (EPIDs). For example, U.S. Pat. No. 7,289,101, titled “Multi-Color Electrophoretic Image Display”, which issued on Oct. 30, 2007, and is assigned to CopyTele, Inc., is illustrative of an EPID. The EPID includes a plurality of cells, each containing electrophoretic particles, capable of displaying different colors when the particles in the cell move from a first position (e.g., rest) to a second position (e.g., display) in the cell. An electrode is coupled to each of the cells and is operative to move the particles from the first position to the second display position (or from the second position to the first position) when properly biased. In this EPID, the electrophoretic particle-containing cells are in fluid communication with one-another. In other types of EPIDs the particles are contained within sealed cells. The cells, whether sealed or partially open, (also referred to as a pixel) may be in a round, a square, a rectangular and/or honeycomb shape or other similar shapes that allow for maximizing the number of cells per unit area (e.g., hexagon, octagon).
The electrophoretic particles used in EPIDs may comprise light colored (light reflective) and/or dark colored (light absorbing) dielectric particles that are suspended in an optically contrasting clear fluid medium (suspension liquid). For example, U.S. Pat. No. 6,113,810, titled, “Methods Of Preparing Electrophoretic Dispersions Containing Two Types of Particles With Different Colors and Opposite Charges,”, and assigned to CopyTele, Inc., describes a dielectric dispersion for use in a electrophoretic display that includes a dielectric fluid, a first plurality of particles of a first color having a surface charge of a selected polarity dispersed within the dielectric fluid and a second plurality of particles of a second color having a surface charge of opposite polarity to that of the first plurality and a steric repulsion thereto preventing coagulation of the first and second particles. As understood by those skilled in the art, the electrophoretic particles described herein may have optical properties that extend from totally light reflective (i.e., white) to totally non-reflective, light absorbing or opaque (i.e., black). Thus, reference to light colored particles refers to particles that have a greater light reflective property than a light absorbing property and dark colored particles refers to particles that have a greater light absorbing property than light reflecting property.
In accordance with the electrophoretic effect described above, the electrophoretic particles in the suspension liquids (fluid medium) selectively migrate to, and impinge upon, a transparent screen electrode, thereby displacing the fluid medium from the screen and creating the desired image.
EPIDs have many advantages over other types of flat panel displays. For example, EPIDs are composed of materials that are relatively inexpensive, and thus, the EPIDs are less costly to manufacture. Another advantage is that the image formed on the screen remains even when power is removed. When the electrophoretic particles or dye particles move to form an image, the image will not erase and remains on the display even after power is removed. Thus, the images created by EPIDs do not have to be refreshed as is necessary for images produced by Liquid Crystal Displays (LCDs) and other types of displays.
However, because an image is created by the movement of the electrophoretic particles within the fluid when the display is properly biased, the response time to view an image is dependent upon the time it takes the particles to move from a rest position to a display position.
Hence, there remains a need for an EPID with a faster response time that provides gray scale imaging that can be manufactured for low cost, and methods for manufacturing same.